This invention relates to medical diagnostic imaging systems, and more particularly, to apparatus and methods for providing highly-versatile diagnostic medical imaging systems capable of performing radiographic and fluoroscopic examinations. Still more particularly, the invention relates to radiographic/fluoroscopic imaging systems, and associated methods, which achieve rapid transitions between fluoroscopic and radiographic modes, and which employ information regarding detected, forecast, or requested motion of the patient or the imaging system in order to reduce the x-ray dose delivered to the patient and examiner.
Medical imaging systems capable of performing both fluoroscopic and radiographic examinations have become highly valuable diagnostic tools in modern radiology. An advantageous application of the dual capabilities of such imaging systems is peripheral angiography. Peripheral angiography is a diagnostic roentgenographic procedure providing visualization and recording of the blood vessels in the peripheral region of the body, such as the arms and legs. In a typical peripheral angiography examination, a radiopaque contrast agent is injected into a blood vessel, and a rapid sequence of radiographs are taken to observe the progress of the contrast agent as it flows through the vessels along the length of the extremity. The contrast agent is initially concentrated in the blood vessels and takes some time to diffuse generally into the surrounding regions. Thus, the contrast agent renders the blood vessels visible under radiography provided that the radiographs are taken very soon after the contrast agent arrives in a particular region.
In conventional Peripheral Angiography examinations, the patient is supported on a movable table top positioned under system control, The table top, in turn, is supported by a stationary radiographic-fluoroscopic table. An overhead X-ray source (which may be mounted on a tube crane) directs a beam through the patient to a "rapid film changer" device.
The locations of interest at any particular time during the examination are in the general vicinity of the leading edge of the contrast material as it progresses though the extremity. In conventional peripheral angiography systems, the rapid film changer is normally in a fixed position. Because the length of the recording radiographic film or imaging device is not sufficient to cover the entire extremity, conventional peripheral angiography systems require that the patient be rapidly repositioned throughout the procedure to fully visualize and record the contrast material as it progresses through the vessels of the extremity (i.e., the patient must be rapidly repositioned throughout the procedure to maintain the contrast material within in the field of view of the rapid film changer). In such conventional systems, the patient rests on a movable table-top, which may travel as rapidly as 9 in/sec between exposures.
Peripheral angiography is representative of mixed fluoroscopic/radiographic examinations in which the examiner, while conducting a fluoroscopic examination, desires to immediately perform a radiographic exposure of a feature or event observed on the fluoroscope. For example, when a radio-opaque dye reaches a certain position in the patient, or some other event of interest occurs during the fluoroscopic examination, it is desirable to immediately record a high-quality radiographic exposure for later use.
In conventional equipment of the type heretofore described, a mechanical operation is required in order to change from the fluoroscopic mode of operation to the radiographic mode, and vice versa. The positions of the radiographic imaging receiver (typically film) or the fluoroscopic imaging receiver (typically an image intensifier) must be exchanged, or an overlapping one of these components must be moved to expose the other. This mechanical operation, even when driven under automatic control of the imaging system, may take one to several seconds. Other time-consuming activities, such as changing certain X-ray tube operating parameters, are also required to perform the transition. However, these activities generally take less time than the mechanical operation and because they are started in parallel, they complete earlier. Accordingly, in older radiographic/fluoroscopic imaging systems, this mechanical operation has been the rate-limiting step controlling the speed at which transitions between radiographic and fluoroscopic imaging modes can be achieved.
Recently, however, filmless radiographic/fluoroscopic imaging systems have been developed which use a single image intensifier (or "photospot") device to receive and record image information during both fluoroscopic and radiographic exposures. As a result, it is not necessary to change film between exposures, nor is it necessary to perform other mechanical operations in order to change between fluoroscopic to radiographic imaging modes because there is no need: move one component out of the way of another. With the elimination of mechanical operations, changing the operating current of the X-ray tube has become the rate-limiting step controlling the speed of transitions between radiographic and fluoroscopic imaging modes in filmless radiographic/fluoroscopic imaging systems.
For a particular X-ray tube employed in an imaging system, the X-ray output delivered by the X-ray tube is directly proportional to the X-ray tube current (which is typically measured in milliamperes (mA)), and is approximately proportional to the fifth power of the X-ray tube voltage (which is typically measured in kilovolts (kV). X-ray tube voltage is selected for the best image contrast, depending on the type of tissue being examined and the character of the examination.
In general, fluoroscopic exposures employ relatively low average X-Ray tube current (e.g., 0.5-3 mA (average)) over a long exposure time, while radiographic exposures use high X-Ray tube current (e.g. 100-1000 mA) over a very short time. The X-ray tube cathode operates by thermionic emission. The X-Ray tube current (i.e., the current flowing between the anode and the cathode) is a function of X-Ray tube anode-cathode voltage (or "high voltage"), X-Ray tube cathode (filament) temperature (which itself is a function of X-Ray tube filament current), and perhaps other factors. However, X-Ray tube current (for a particular selected high voltage) is generally controlled by adjusting the filament temperature, which, in turn, is controlled by adjusting the filament current.
X-Ray tubes which are suitable for both fluoroscopic and radiographic exposures may include one or two filaments of differing sizes. Where a single filament is used, and it is desired to change from fluoroscopic to radiographic mode, the filament current must be increased to allow the filament temperature to increase, thereby permitting a higher X-Ray tube current which is sufficient for radiographic exposures. Where two filaments are provided, one filament is typically kept at a standby temperature just under the cathodic emission temperature, to avoid deterioration of the filament, except when the filament is selected for use. Thus, even for two-filament tubes, when a radiographic exposure is desired, the radiographic filament current must be increased to allow the filament to heat to a sufficient temperature.
Because it takes time to heat or cool the filament to a desired temperature, the X-ray tube current cannot be instantaneously controlled. It typically takes around one second for the filament to heat from an initial temperature (such as its temperature when operating in fluoroscopic mode or when in standby) to the temperature required for a radiographic exposure. In conventional radiographic/fluoroscopic systems, the high-voltage power supply to the tube is disabled, thereby inhibiting X-Ray emission, during the filament heating period. Thus, the radiographic exposure does not begin until after the filament reaches the required temperature. This delay can be significant, because the dye may progress a substantial distance, or a transient event may have ended, before the radiographic exposure can be recorded.
The opposite transition, from radiographic mode to fluoroscopic mode, is equally important. In radiographic mode, the system operates with relatively high x-ray tube current. Tube current is a function of the temperature of the cathode or filament, and therefore, in radiographic mode, the filament must be relatively hot to support the high required tube current. In fluoroscopic mode, much lower current, and therefore, accordingly-reduced filament temperature, is typically used. The cooling of the filament is an exponential process over time, so that the tube current cannot be instantaneously reduced to the desired level normally used for fluoroscopy. In conventional radiographic/fluoroscopic imaging systems, in which a transition from radiographic to fluoroscopic mode is desired, the system must wait for the filament to cool down to a temperature appropriate to produce the tube current desired in fluoroscopic mode. This delay is undesirable.
Another problem with prior art radiographic/fluoroscopic imaging systems is that they do not optimally minimize the radiation dose delivered to the patient (and radiologist, technician, or other examining personnel) during an examination. For example, in fluoroscopic examination systems, fluoroscopic exposures may be made continuously, at low X-ray tube current (mA), or in short, repetitive bursts or pulses, at higher tube current. In pulsed fluoroscopy, digital video memory is used to preserve the displayed image between pulses. For a selected average x-ray dose as continuous fluoroscopy, the momentary X-ray tube current is higher, resulting in higher signal-to-noise ratio.
Pulsed fluoroscopy systems may have low and high pulse repetition rates. Lower pulse repetition rates are desirable in that they result is a lower accumulated radiation dose to the patient, and any other personnel in the vicinity. When an observed scene is stationary, low-rate pulsed fluoroscopy is preferred because it results in a lower dose to the patient and the operator. However, if movement or changes occur in the observed scene, the changes appear only when the exposure pulses occur. At low repetition rates, brief transient events may be missed entirely, and movement appears jerky. It has been noted that even in radiographic/fluoroscopic imaging systems which allow the examiner to vary the pulse rate in response to patient motion, examiners often use a high pulse rate (appropriate for observing movement) throughout the examination, including those periods in which no movements or changes in the image are actually occurring or expected. This undesirably increases the radiation dose delivered to both the patient and the examiner.
Nields U.S. Pat. No. 5,119,409 discloses a pulsed fluoroscopy system which analyzes the fluoroscopic image and responsively dynamically controls the fluoroscopic pulse rate based on motion detected in the image. This system has the disadvantage that the fluoroscopic image cannot be acquired without exposing the patient to X-rays.
Another disadvantage of prior art radiographic/fluoroscopic systems is that they employ error-prone methods of determining when to initiate a radiographic exposure. The peripheral angiography examination described above is an example of a type of imaging examination to which modern imaging systems are applied in which the patient undergoes a continuous or repetitive-pulse fluoroscopic examination while the examiner awaits an event of particular interest. The event may be, for example, movement of the patient (as might occur as the patient swallows or breathes), or the arrival of a contrast medium or dye in the image or at a particular location in the image. The occurrence of the event may then trigger the desire to perform a radiographic examination, which may range from a single radiographic (or "photospot") exposure to a preprogrammed sequence of radiographic exposures and movement of the patient interspersed such that the exposures occur at various patient locations.
In prior-art radiographic/fluoroscopic imaging systems, a radiologist or technician must observe the fluoroscopic display to detect the event of interest, and then initiate the radiographic exposure (and in most cases, each individual radiographic exposure thereof). This means that the observer must have extensive training and experience and must employ careful, vigilant observation. If the radiographic examination is initiated too early or too late, or the event is missed, the results of the examination may be of poor quality or may be entirely useless; re-examination is undesirable because the patient receives additional exposure to radiation.